Jean-Claude Bunzli - Programming heteropolymetallic lanthanide helicates: Thermodynamic recognition of different metal ions along the strands

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  Floquet,S., Borkovec,M., Bernardinelli,G., Pinto,A.,   Leuthold,L.A., Hopfgartner,G., Imbert,D., Bunzli,J.C.G.,   Piguet,C. Chemistry-A European Journal  2004 10 (5) 1091-1105

  Abstract: Under stoichiometric conditions the   segmental tris-tridentate ligand L9 assembles with two different   lanthanide metal ions Ln(1) and Ln(2) (Ln(1), Ln(2) = La, Nd, Sm,   Eu, Yb, Lu, Y) to give mixtures of the heterotrimetallic   triple-stranded helicates [(Ln(1))(x)(Ln(2))(3-x)(L9)(3)](9+)   (x=0-3) in acetonitrile. The combination of qualitative (ESI-MS)   and quantitative (H-1 NMR) speciations provides a set of   thermodynamic data that were analysed with various statistical   chemical models. A satisfying description requires the   consideration of different affinities for the terminal N6O3 sites   (k(Ln)(t))and for the central N-9 site (k(Ln)(c)) for each   specific lanthanide. The nontrivial dependence of these   parameters on the ionic radius provides size-discriminating   effects that favour the formation of heterotrimetallic helicates   in which the central site is occupied by the larger metal of the   pair. Combining the latter enthalpic driving forces with entropic   contributions due to specific stoichiometric conditions allows   partial selection (i.e., programming) of a specific   heterotrimetallic species in solution, which can be isolated by   crystallisation, as demonstrated for [Eu2.04-La0.96(L9)(3)]   (CF3SO3)(9)(CH3NO2)(9) (1, Eu2.04La0.96C207H222N48O51S9F27,   monoclinic, P2(1)/c, Z = 4) in which the cation   [EuLaEu(L9)(3)](9+) is the major component in the crystal. The   scope and limitation of this approach is discussed together with   the conditions for explicitly considering intermetallic   interaction parameters u(LnILn2) in more sophisticated chemical   models

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